Walk around the room right now and pay attention to your arms. No doubt they're dangling straight at your sides. But if you were to pick up the pace and start jogging, you'd likely bend your arms at the elbow.

Running with your arms bent and walking with your arms straight is what most of us do, but until recently, scientists didn't know why.

To find out, Andrew Yegian and his fellow researchers at Harvard University had eight volunteers walk and run on a treadmill with their arms straight and bent. Six of the participants wore oxygen masks to record how much oxygen they used. Researchers tracked all the volunteers' movements using reflective markers on their shoulders, wrists and elbows, according to The Guardian.

The team predicted that it was more efficient to have straight arms while walking and bent arms while running, but they were only half right.

They found that walking with arms bent used more energy than walking with straight arms, increasing oxygen consumption by 11%. But surprisingly, running with bent arms was no more efficient than running with straight arms.

"Pretty much every subject in the study said that straight-arm running was the most challenging condition," Yegian told New Scientist. "That’s why it was very surprising when we couldn’t find any difference in the energetics.”

The study, which was published in the Journal of Experimental Biology, involved students who ranged from marathon runners to those who ran just a few times a week.

There has to be a reason

The researchers say they don't know why we usually bend our arms when we run. Yegian says he thinks there has to be some beneficial reason. It could be because bending the arms helps stabilize the head.

And because the research was only conducted at low running speeds, the benefit of arm bending may be more obvious at higher speeds.

Yegian plans to do more testing to find out.

He told The Guardian, "We have a pretty good idea now that energy is why we keep our arms straight when we’re walking and there’s probably a specific reason for bending your arms during running."

Mary Jo DiLonardo covers a wide range of topics focused on nature, health, science and anything that helps make the world a better place.

Why do we walk with straight arms but run with our arms bent?
Researchers looked at walkers and runners with bent and straight arms to find out.